Methods for the quantification of nucleic acids are important in many areas of molecular biology and in particular for molecular diagnostics. At the DNA level, such methods are used, for example, to determine the presence or absence of variant alleles, the copy numbers of gene sequences amplified in a genome, and the amount, presence, or absence of methylation across genes or at specific loci within genes. Further, methods for the quantification of nucleic acids are used to determine mRNA quantities as a measure of gene expression.
Among the number of different analytical methods that detect and quantify nucleic acids or nucleic acid sequences, variants of the polymerase chain reaction (PCR) have become the most powerful and widespread technology, the principles of which are disclosed in U.S. Pat. Nos. 4,683,195; 4,683,202; and 4,965,188.
Detection of nucleic acids that are present at low levels in samples (e.g., such as DNA from a disease locus, e.g., a tumor, that is collected from a sample that is remote from the disease locus, e.g., DNA that finds its way into stool, sputum, urine, plasma, etc., “remote DNA samples”) can be difficult, in part because many DNAs found in such samples are not only present in low amounts, they are also generally fragmented. See, e.g., WO 2006/113770 to Ballhause, and US Patent Publication US 201110009277 A1, to Davos, each of which is incorporated herein by reference in its entirety. For example, cell-free DNA (cfDNA) found in plasma can be highly fragmented, and much of the DNA that might be of interest, e.g., tumor-derived DNA can be very small, e.g., 200 or fewer nucleotides in length. Nucleic acids of this size can be lost during routine purification, due to, e.g., poor binding to purification columns or inefficient alcohol precipitation.
Analysis of such nucleic acids from such samples is especially difficult if multiple targets or loci in the nucleic acid(s) need to be detected. For example, a collected specimen having small numbers of copies of the targets of interest often cannot be divided into a sufficient number of aliquots to permit testing for all targets without risking the accuracy of the tests for the individual targets, e.g., by false negative results.
Pre-amplification of target nucleic acids (e.g., genomic DNA, cDNA, etc.) in a low-target sample may be used to enrich the DNA in the sample prior to dividing the sample for further specific target analysis. For example, whole genome amplification using simple primers (e.g., random hexamers) has been used to increase the amounts of essentially all DNA in a sample, in a manner that is not specific to any particular target of interest. (Sigma-Aldrich's GenomePlex systems, Arneson, et al., Cold Spring Harb. Protoc.; 2008; doi:10.1101/pdb.prot4920).
Another approach is to amplify one or more regions of particular interest in a semi-targeted manner, to produce a mixture of amplified fragments (amplicons) that contains the different mutations or loci that will be further analyzed. Successive rounds of amplification using the same primers are prone to high background of non-specific amplification, and the production of artifacts, e.g., artificially recombined molecules, high non-specific background, and biased amplification of different intended targets. Thus, such pre-amplification PCR is typically carried out under special conditions e.g., a limited number of cycles, and/or using a low concentration of primers (e.g., 10 to 20-fold lower than in standard PCR) to avoid increases in non-specific background amplification, as use of concentrations over about 160 nM of each primer in multiplex pre-amplification has been shown to increase amplification background in negative control reactions (see, e.g., Andersson, et al., Expert Rev. Mol. Diagn. Early online, 1-16 (2015)).
After a first round of amplification in a multiplex PCR, pre-amplified DNA is typically diluted and aliquoted into new amplification reactions for quantitative or qualitative PCR analysis using conditions typical of standard PCR, e.g., higher concentrations of reagents and larger numbers of cycles, and the second amplification is generally carried out using different primer pairs, e.g., “nested” primers that anneal to sites within the pre-amplified fragments, rather than annealing to the original primer sites at the ends of the amplicons.
When DNA is to be examined for methylation, the analysis is further complicated by the fact that commonly used processes for preparing samples for methylation detection typically result in substantial losses of sample DNA. For example, bisulfite treatment is typically used to convert unmethylated cytosine residues to uracil residues, but the process typically results in only about 30% recovery of the input DNA. In addition, amplification of DNA after treatment with bisulfite is especially challenging. For example, the conversion of unmethylated cytosines reduces the complexity of the DNA sequences and the treatment itself is known to cause significant damage to the DNA, e.g., strand breakage, both of which can contribute to increased background in amplification reactions, especially in multiplexed amplifications.